An air charge temperature sensor is a device inside the engine that monitors the temperature of the air going into the engine. In cars made after 1995, this part is called the intake air temperature sensor. The sensor sends temperature data to the car engine's computer for air density estimations used to maintain a proper mixture of air and fuel.
A car's computer needs an intake air temperature sensor to send it data regularly, as different temperatures of air have different density levels. The computer uses the timing of the injector pulses to maintain the proper air-to-fuel ratios at the various densities.
This sensor is most commonly found on the intake manifold with its tip exposed to monitor the air entering the engine. If the engine is designed with a split or separate intake manifolds, there is a sensor for each one.
Due to its location, the intake air temperature sensor can be damaged if the intake manifold backfires. Other factors that damage its functionality include oil and carbon contamination and degradation due to heat and age. In either of those circumstances, the sensor becomes less responsive or ceases to function altogether. Poor electrical connections also prevent data transmission from the sensor to the computer.